An inkjet printing system has a low level of noise and low running cost, and can realize high-speed printing, easily downsize a device for use, and is easily applied for color printing. Because of these advantages, this system has been widely used in printers, photocopiers, and the like. Inks for use in such printers are usually selected under the consideration of the printing properties such as jetting performance, and fixing ability, and print qualities such as occurrences of blurring in an printed image, optical reflection density of the image, and coloring ability.
It has been widely known that inks can be roughly classified into two types, i.e. dye inks and pigment inks, depending on the coloring material contained in the ink. The pigment inks have excellent water resistance and light resistances compared to the dye inks and also have an advantage that printing of vivid and clear characters is possible. However, the pigment inks have downsides such that the pigment inks require longer time to be fixed onto a printing medium than the time required for fixing the dye inks, and the resulting fixed images formed by the pigment inks may not have sufficient abrasion resistance. Moreover, there is a tendency that the size of dots formed on a printing medium with the pigment ink jetted from a nozzle by one jetting operation becomes small.
A pigment contained in a pigment ink is generally stably dispersed in the ink by using mainly electric repulsive force of a polymer dispersing agent to break the intermolecular forces between pigment particles. Note that, these intermolecular forces cause aggregations of the pigment particles. For this reason, the polymer dispersing agent is preferably added to the ink depending on the amount of the pigment contained therein.
When such ink is jetted onto a piece of plain paper for printing using an inkjet recording system, the pigment particles contained therein are aggregated to each other along with penetration of the solvent (e.g. water) of the ink into the paper, and evaporation of the solvent to the air. As the behavior of the ink on the paper at this time, the aggregation force of the ink increases as the amount of the polymer dispersing agent contained in the ink increases. Due to the aggregation, a size of a dot formed on a printing medium by the ink having a certain volume and jetted from the inkjet head decreases. In addition, the formed dot has a shape, which remains the obscured shape of the ink when the droplet of the ink was hit on the paper with an impact. To form ink dots having dot diameters necessary for recording to have sufficient recording density to form an image, and not to cause white-out lines, therefore, it is necessary to adjust the volume of the jetted ink from the inkjet head to relatively large. Even through such adjustment is performed, strong aggregation forces of the pigment particles to each of which the polymer dispersing agent has been adsorbed reduce the penetration performance of the ink into the paper, to thereby slow the fixing speed of the ink to the printing medium or decrease the abrasion resistance of the resulting recorded image.
For the purpose of enlarging the dot diameter and improving the penetration of the ink into the printing medium to thereby improve the fixing ability, there has been a proposal that a penetrating agent is added to an ink. However, such ink may cause undesirable phenomena for the production of high quality recording images. For example, the dot shape may be degraded (i.e. degradation of the shape surrounding the dot, such as feathering), and the ink may be passed through to the back side of the paper (i.e. back side bleeding). Since the coloring agent penetrates into the inner side of the printing medium, the optical density (OD) of the formed ink dot does not increase though the dot diameter is relatively large.
In the coming future, the application of inkjet printers for business use will be actively developed. In such circumstance, it is expected that there is a demand for further improvement of printing speed with inkjet printer. If the fixing ability of an ink to a recording medium is insufficient in this case, the following problem may occur. For example, when the printed recording mediums are successively discharged from the inkjet printer and successively stacked, a second recording medium discharged following to the previously discharged first recording medium is stacked on the printed surface of the first recording medium before the ink deposited on the printed surface of the first recording medium is fixed to thereby disturb the image formed on the printed surface of the first recording medium, or smear the back side of the second recording medium with the ink of the first recording medium.
Furthermore, there has been a proposal of an ink in which a self-dispersing pigment is used. With this ink, dot diameters can be increased, probably because the aggregation force of the self-dispersible pigment is weaker on the paper than the pigment dispersed in an ink with assistance of the polymer dispersing agent. However, this ink is not still sufficient to resolve the aforementioned problems.
Accordingly, various researches and developments need to be made in a printing method satisfying, at high level, the various elements, which may determine the quality of the resulting recorded images, such as fixing ability of the ink, enlargement of the ink dot diameter, uniformity of the density within the ink dot, and high optical density of the ink dot itself.
For the purpose of further improving printing qualities and image qualities in an inkjet printing technology (e.g. improvements of water resistance or optical density (OD) of an image on a printing medium), there has been a proposal of a method in which an ink and a processing liquid, which reacts with the ink on a printing medium, are applied on the printing medium so that the ink and the processing liquid are reacted. This proposal has been also used in practices. For example, as a method for providing a high quality (e.g. no feathering and no bleeding) print, there is a proposal of a method (see PTL 1) in which a self-dispersible pigment to which anionic atomic groups are bonded, a self-dispersible pigment to which cationic atomic groups are bonded, and a processing liquid are each defined. Another proposal for this is a method (see PTL 2) in which as a coloring agent not containing a surface-based dispersing agent for a black ink, for example, a coloring agent of self-dispersible type, polymer emulsion type, anionic compound, or water-soluble dye is contained in the black ink, and a color ink is specified to contain a coloring agent containing a surfactant-based dispersing agent. Yet another proposal is a method (see PTL 3) in which a black ink is specified to contain a cationic self-dispersible pigment, and a color ink is specified to contain an anionic dye and additives.
However, these methods have problems. For example, aggregations are caused within an ink when colors of inks are mixed on a head water-repellent membrane, to thereby deposit on the water-repellent membrane, and cause jetting failures. Moreover, the ink using the cationic self-dispersible agent has poor stability over time. In the case where the dye is used in the ink, high quality (e.g. no feathering, no bleeding) prints are not easy to obtain compared to the case where the ink containing the pigment is used. Accordingly, it is a current situation that an inkjet recording ink set, which satisfies desirable jetting performance, formation of high quality images, and stability over time, has not yet been provided.